Lipid Oxidation in Fillets of Herring (Clupea harengus) during Processing and Storage
The influence of storage and processing on the development of lipid oxidation in herring fillets was investigated and related to their composition of lipids, catalysts and antioxidants. The progress of oxidation was monitored as antioxidant loss and as the development of primary, secondary and tertiary lipid oxidation products.
To reduce the toxicity of the lipid extractions, the traditional chloroform/methanol /water system was compared with three alternative alkane/alcohol/water-based systems. The main point of difference was the phospholipid (PL) yields obtained, which became significantly lower with alkane/alcohol/water. However, it became possible to use the system comprising heptane/ethanol/sodium dodecyl sulphate (SDS)/water by decreasing the SDS-concentration. This modification increased the PL yields to same levels as with chloroform/methanol/water.
Ice storage of skin-on herring fillets for up to 15 days gave significant changes in both oxidation products and antioxidants within 2-3 days, which yielded "rancid odour" after only 2.5 days. No significant rise in total bacteria was seen until after 7 days. Comparison of different horizontal layers of the fillets showed that the under skin layer oxidised significantly faster than the middle and inner parts, which was supported by its much higher aqueous pro-oxidative activity. Keeping the herring fillets on ice for more than 3 days significantly increased the oxidation rate during subsequent frozen storage. Among the fillets kept for 0, 3, 6 and 9 days on ice, oxidation products were observed to form most rapidly in the 6-day fillets.
Large differences were found in the oxidation rates in different parts of the fillets during frozen storage, which were ascribed to differences in composition and in oxygen access. When dark muscle, light muscle and skin were stored separately from each other, the three tissues oxidised according to: dark muscle>skin >light muscle, which was the same ranking order as was found for levels of iron, copper and aqueous pro-oxidative activity. However, when the tissues were stored as an intact fillet, the oxidation order changed to: skin>dark muscle>light muscle, indicating that the skin and light muscle protected the dark muscle e.g. from oxygen. The skin also significantly suppressed oxidation in the under skin layer of the fillet, and, to some extent, also in the middle part. However, in both skin-on and skinned fillets, the under skin layer oxidised significantly faster than the inner and middle parts.
An unexpected finding was that mincing the fillets improved the oxidative stability at -18 °C. In comparison with intact skinned fillets, the oxidation rate in the outer 4 mm of the herring mince samples was 3-4.8 times lower, and in the centre of the samples, 20-25 times lower. Washing the mince, which reduced the levels of total fat, neutral lipids iron, copper, selenium, glutathione peroxidase activity and aqueous pro-oxidative activity, decreased the storage stability. Antioxidative properties of the washing water residue revealed that more antioxidants than catalysts were washed out. Washing also appeared to induce lipolysis. Pre-cooking of the mince samples at 55 °C diminished oxidation, most likely due to inactivation of catalytic enzymes. The opposite was found after pre-cooking at 100 °C, which was ascribed to haemoprotein denaturation. At both 55 and 100 °C, rised cooking time from 38 to 54 min negatively affected the stability. Washing prior to the pre-cooking diminished the beneficial effects of mild heating.